A radio signal transmitted by an antenna of a communicating entity is subject to distortion as a function of the propagation conditions between a source point defined at the output of the source antenna and a destination point defined at the input of an antenna of the destination communicating entity. To limit this distortion, the antenna signal is predistorted by applying pre-equalization coefficients as a function of the characteristics of the propagation channel between these two antennas. It is therefore necessary to characterize this propagation channel.
Of existing pre-equalization methods, methods using time reversal are distinguished by their reduced complexity, their performance, and their intrinsic capacity for focusing a radio wave onto a receive antenna. Time reversal makes it possible to reduce significantly the temporal dispersion of the propagation channel by focusing the energy of the received signal in time and in space.
Time reversal is a technique for focusing waves, typically acoustic waves, that relies on the invariance of the wave equation on time reversal. Thus a time-reversed wave propagates like a forward wave traveling back in time. A short pulse emitted from a source point propagates in a propagation medium. Part of this wave received by a destination point is time reversed before it is sent back in the propagation medium. The wave sent back converges toward the source point, where it forms a short pulse, and the energy of the wave is focused on the source point. The shape of the signal focused on the source point by time reversal is virtually identical to that of the source signal emitted at the source point. Thus time recompression occurs at the source point. In particular, the more complex the propagation medium, the more precisely the time-reversed wave converges.
The time-reversal technique is therefore applied in radio communications networks to cancel the effect of the propagation channel on the signal, notably by reducing the spreading of the channel, and to simplify the processing of symbols received after passing through the channel. The antenna signal emitted by an antenna of the source communicating entity is thus pre-equalized by applying coefficients that are obtained by time reversing the impulse response of the propagation channel that this signal has to pass through. The time reversal of the propagation channel applied to the signal makes it possible to cancel the effect of this channel on transmission from the source point of the signal predistorted in this way and to focus the signal on a destination antenna. Time reversal therefore requires the source communicating entity to have knowledge of the propagation channel.
However, if the communicating entities are mobile, the channel estimate produced at a given time by a communicating entity may prove erroneous at a later time because of the mobility of the communicating entities. This error is qualified relative to a defined relative movement as a function of the movement of the source communicating entity as observed from the destination communicating entity. For large relative movements there is decorrelation between the estimate of the propagation channel used to predistort the signal and the propagation channel that the signal actually passes through. The signal is not focused on the antenna of the destination communicating entity and the power of the received signal is lower than the power of the signal that would be received in the absence of relative movement.
The defocusing of the signal at the destination antenna is in fact generated by the relative movement and by a latency delay. The latency delay is the overall delay including a processing delay between measurement of the propagation channel and transmission of the pre-equalized signal and a delay between transmission and reception of the pre-equalized signal.
The position of the destination antenna relative to the estimate of the propagation channel applied to the pre-equalized signal defines a focal point. The focal point therefore corresponds to the position of the destination antenna relative to the source antenna before the latency delay. In other words, the focal point corresponds to the point of convergence of the waves with stationary communicating entities and no latency delay. If one or both of the two communicating entities moves during the latency delay, the destination antenna is at a position that is different from the position of the focal point relative to the source antenna, and the focal point is therefore not respected.
If the signal is not focused, the quality of service is degraded. The quality of service is for example the data rate offered or an error rate for the data conveyed by the signal. Thus the quality of service deteriorates as soon as the destination communicating entity and/or the source communicating entity moves. This deterioration increases as the movement of the source communicating entity relative to the destination communicating entity during the latency time increases. In other words, the power received at the destination antenna decreases if the relative movement increases during the latency time. Moreover, this deterioration is a function of the carrier frequency of the pre-equalized signal.
It is therefore necessary to qualify focusing quality.